1. Field of the Invention
The present invention relates to a digital filter capable of computing a tap without output delay due to the filter operation in a symbol time, and a digital broadcasting receiver having the same.
2. Background of the Related Art
A digital filter based on the LMS (Least Mean Square) adaptive algorithm is a filter capable of updating or adapting coefficients on an ongoing basis. The LMS adaptive digital filter is usually used for an equalizer or a noise eliminator housed in a digital broadcasting receiver, in order to compensate the distortions generated by a channel or a system itself.
The LMS adaptive digital filter includes a multiplier and an adder for the coefficient adaptation for each tap, and an additional multiplier for the output (filtering).
FIG. 1 illustrates the general structure of an LMS adaptive filter, more particularly, a 2-tap LMS adaptive filter. As shown in FIG. 1, the LMS adaptive filter includes four serial delays D11, D12, D21 and D22, D11 and D12 for delaying an input signal x0 in sequence and D21 and D22 for delaying a delayed input signal xd0, and a first and a second coefficient updating unit 10, 20.
Each of the delays D11, D12, D21 and D22 operates according to a clock (clk) signal, and a first and a second tap, i.e., the first and the second coefficient updating unit 10, 20 have the identical structure.
Here, the input signal x0 is outputted to the delay D11 and at the same time to a multiplier 14 of the first coefficient updating unit 10. The delay D11 delays the input signal x0 by one clock, and outputs the one-clock-cycle delayed signal to the delay D12 and a multiplier 24 of the second coefficient updating unit 20 at the same time.
The delayed input signal xd0 is simultaneously outputted to the delay D21 and a multiplier 11 of the first coefficient updating unit 10. The delay D21 delays the delayed input signal xd0 by one clock, and outputs the delayed signal simultaneously to the delay D22 and a multiplier 21 of the second coefficient updating unit 20. The delay D12 delays the delayed signal x1, which was delayed by the delay D11, by one clock before outputting the signal. The delays D22 delays the delayed signal xd1, which was delayed by the delay D21, by one clock before outputting the signal.
The multiplier 11 of the first coefficient updating unit 10 multiplies the delayed input signal xd0 by a feedback error signal e, and outputs the result to an adder 12. The adder 12 adds an old coefficient c0 to the output from the multiplier 11 for the coefficient update, and outputs the updated coefficient to a delay 13. The delay 13 delays the updated coefficient in the adder 13 by one clock, and outputs the delayed coefficient to the adder 13 and the multiplier 14. The multiplier 14 then multiplies the output from the delay 13 by the input signal x0 to obtain a first output y0.
The multiplier 21 of the second coefficient updating unit 20 multiplies the delayed input signal xd1 by a feedback error signal e, and outputs the result to an adder 22. The adder 22 adds an old coefficient c0 to the output from the multiplier 21 for the coefficient update, and outputs the updated coefficient to a delay 23. The delay 23 delays the updated coefficient in the adder 23 by one clock, and outputs the delayed coefficient to the adder 23 and the multiplier 24. The multiplier 24 then multiplies the output from the delay 23 by the input signal x1 to obtain a second output y1. That is, the outputs y0 and y1 are obtained by multiplying the input signals (x0, x1) by the coefficients (c0, c1) for each tap, respectively.
Recently long-term fading channels are often found because of temporally distant media like ground wave digital TVs. Thus the fading problem should be resolved to facilitate the broadcast receiving operation. However, to compensate the long-term fading by the temporally distant media a multi-tap equalizer or a noise eliminator.
Unfortunately though, the size of the multi-tap filter is so big that the implementation of the filter becomes difficult.